1. Field of the Invention
The present invention relates to an ignition control system for an internal conbustion engine in which ignition timing is controlled by a microcomputer.
2. Description of the Related Art
In a known ignition control system for an internal combution engine, when the engine is operating, the rotational position of the crankshaft is first sensed by a crank angle sensor which is mounted on a camshaft of a distributor, the crank angle sensor outputting a signal representative of the sensed crankshaft rotational position to a microcomputer including a CPU, a RAM and a ROM so that the signal is temporarily stored in the RAM. An intake pressure sensor senses the intake pressure in the intake pipe or manifold of the engine representative of the engine load condition and outputs a signal representative of the sensed intake pressure to the CPU through an analog to digital (A/D) converter, which converts the output in the form of an analog signal of the intake pressure sensor into a digital signal, which is temporarily stored in the RAM. The CPU calculates the number of revolutions per minute of the engine based on an ignition period which is obtained through the crank angle sensor by using a calculation program previously stored in the ROM. Based on the number of revolutions per minute of the engine thus calculated and the output signal from the intake pressure sensor representative of the engine load condition, the CPU determines a target ignition timing from an ignition timing map previously stored in the ROM. The target ignition timing is then fed to an ignition device to turn on and off a power transistor incorporated therein for temporarily interrupting the current supply to the primary side of an ignition coil so that a high voltage is thereby generated on the secondary side of the ignition coil, thus causing a spark plug to electrically spark and ignite the air/fuel mixture in the corresponding engine cylinder.
With the above-mentioned ignition control system in which ignition timing is determined based on the output signal of the crank angle sensor, determination of ignition timing can not be made at all in the case of failure in the crank angle sensor, with the result that the engine is stopped. Further, the mounting position of the crank angle sensor, which is mounted on the camshaft in the distributor by a fastener such as a fastening belt or the like, may be sometimes displaced from the original correct position due, for example, to loosening or slackening of the fastening belt or the like so that the exact rotational position of the crankshaft can not be sensed.